Is it fair that oss auhtors/maintainers are not being contributed to by for-profit companies that rely on their products even though the licence allows?

One of the points in OSS is to make software a commodity. Obviously, this is at odds with directly making a living from it.

However, companies like, Intel, AMD and ARM can sell their products better if Linux plays well with them, that's why they invest in Linux. Means, they pay their emplyoees to contribute.

Other OSS authors are in as hobby, often with the side effect of showcasing something substantial when applying for a job. Hobbies don't have to pay themselves - in fact, most hobbies don't.